Conventionally, there has been well known a steering apparatus for a vehicle which has an auxiliary steering angle superposition mechanism installed at a location between a steering wheel and steerable road wheels for changing a transmission characteristic between the steered angle of the steerable road wheels and the steering-wheel steering angle of a driver in accordance with the traveling condition of the vehicle (see, for example, a first patent document).
In this kind of steering apparatus, when the electric motor becomes a freely rotatable state, even if the driver steers the steering wheel, the steerable road wheels become unable to be steered while permitting only the electric motor to rotate. Accordingly, in order to deal with this problem, in the above-mentioned conventional apparatus, the rotation of the electric motor is inhibited by driving on one-side transistors of upstream side and downstream side transistors of a transistor bridge circuit and driving off the other-side transistors thereof for the purpose of inhibiting the rotation of the electric motor.    [First Patent Document]
Japanese patent application laid-open No. 2005-350036